1. Field of the Invention
This invention relates to impact printers. More specifically, it relates to ribbon feed or supply mechanisms in such impact printers.
2. Description of Prior Art
With the development of the printer field in the direction of high speed impact printers producing high quality printing suitable for correspondence at high speed in the order of 60 cycles per second, new needs have arisen with respect to printer ribbon structure and feed mechanism.
Because of the high throughput of such printer apparatus and the consequently high volume of printed characters, the art has had to provide a ribbon which is of relatively low cost but yet provides high quality printing. Because of the difficulty in meeting these requirements with the more traditional fabric base or carbon film ribbons, the art has been working with a more recent type of ribbon which is a cast matrix of a plastic such as nylon containing liquid ink. While such ribbon structures appear to provide the combination of high quality printing and low cost, they are highly flexible and fragile. Further, they are sensitive to high temperature and high humidity. For example, at temperatures in the order of 25.degree. C. and 80% relative humidity, as little as 30 grams of ribbon tension may cause objectionable yielding and frequent breakage of a cast matrix type of ribbon which is in the order of 0.6 cm. wide.
It consequently becomes very important that the ribbon supply and feed system be operated as free as possible of "drag" of friction acting against the ribbon as it is being removed from the supply spool and fed towards the printer impact point.
Another critical aspect involved in the use of the relatively fragile film type ribbons in high speed printing is that of end of ribbon sensing. The reason for this criticality is that because of the fragile nature of such ribbons, it is necessary to use both ribbon leaders and ribbon trailer of reinforcing but not printable materials in order to avoid ribbon breakage during manufacturing loading and unloading. In a high speed printing operation of the type described, inaccuracies in end of ribbon sensing can often lead to the nonprinting ribbon follower being fed into the print position for up to several characters before the end of the ribbon is finally sensed. This of course is unacceptable in high speed printing apparatus as it leads to nonprinted characters, a condition which is difficult if not totally impractical to remedy.